"# of 50" refers to the frequency entries in the file "[MC installation folder]\TV\DVB Frequencies.xml". Open the file and find United Kingdom.
You do not need to do it that way. You can let MC pick up all channels that can be picked up, and then use channel editing tool to hide those channels that you do not want.
Tools > Options > Television > Advanced > Edit channel list...
Select all channels that you do not need, and click "Hide" button. Note that it is recommended that you use "Hide" button instead of "Delete" button. If for some reason you need to re-run channel scanning (for example, a new channel comes along), all the deleted channels will be picked up again, and you will have to delete them again. With hiding, the unwanted channels will still be picked up during re-scan, but they will remain hidden (unless the channel parameters have changed, in which case they may be picked up as new channels).